In the prior art process which has heretofore been commonly used to prepare tabular AgX grains, nucleation, ripening and growth are effected in one tank (a so-called batch process). This process has two disadvantages.
First, in order to prepare tabular grains having narrowly distributed sizes, the duration of nucleation is preferably short. However, when the duration of nucleation is shortened, the amount of added solutes is reduced, reducing the yield of an AgX emulsion. If the rate of addition is raised in an attempt to make up for the reduction in the amount of added solutes, high concentration solute solutions must be added at a great flow rate. This creates the problem that agitation and mixing speed cannot keep up with the flow rate, particularly in mass production.
Second, in the nucleation and ripening procedures, nuclear grains which have been formed early and grains which have been formed later are mixed with each other, causing physical ripening therebetween and resulting in a wider size distribution. Further, in the nucleation procedure, nuclei which have been formed early undergo physical ripening and grain growth, while those formed later undergo less physical ripening and grain growth. This also widens the size distribution of tabular AgX grains.
In order to eliminate these disadvantages, various improvements have been attempted in the tank process. However, none of these attempts has been successful. For example, JP-A-2-838 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses in its example a nucleation duration of 5 seconds to 10 minutes at a temperature of 30.degree. C. EP 0,362,69A2 discloses the addition of a silver salt solution to the system in 2 seconds to form nuclei. However, such a technique is infeasible in mass production as mentioned above. The aforementioned problems cannot be solved at the same time in the batch process. As a process other than the batch process, European Patent 0408752A1 discloses a process which comprises continuously supplying to a high temperature reaction vessel finely divided grains formed in a mixing vessel provided external to the reaction vessel, and then immediately causing ripening reaction to effect nucleation reaction. However, since nuclei which have been introduced early into the reaction vessel and those which have been introduced later have different histories, and since both these grains are ripened at the same time, uniformity is reduced.
U.S. Pat. No. 5,104,786 discloses adding a silver salt solution to an aqueous solution containing a halide and passing a protective colloid through a pipe to effect nucleation. In this process, it is true that nuclear grains thus formed advantageously have the same history. However, since the aqueous solution of silver salt and the aqueous solution of halide are mixed in a fine pipe, turbulence needs to occur in the pipe, requiring a large amount of liquid to flow in the fine pipe. The mixing in a pipe attains a lower mixing efficiency than that obtained by general agitators. Therefore, the concentration of the solution to be added must be minimized to a relatively low value. This inevitably means that the yield of the emulsion will be reduced. Further, this patent provides no improvement in the uniformity of the history of various grains in the ripening procedure, and the ripening procedure is still batch-wise.